1. Field of the Invention
The present invention relates generally to an active equalizer, and more particularly to an active equalizer by which a signal within a selected frequency range can be either boosted or attenuated with an improved signal-to-noise ratio.
2. Description of the Prior Art
A known active equalizer is shown in FIG. 1 to include a potentiometer 3 having a grounded center tap, with one end of its resistance element being connected to an input terminal T.sub.1 and the other end of the resistance element being connected to an output terminal T.sub.2. An inverting amplifier 4 has an inverting input terminal connected through a first or input resistor 1 to signal input terminal T.sub.1, and an output terminal connected to signal output terminal T.sub.2. The non-inverting input terminal of inverting amplifier 4 is connected to a source of a reference potential, or to ground. A second feedback resistor 2 is inserted between the inverting input terminal and the output terminal of inverting amplifier 4. The known active equalizer also includes a transfer impedance circuit 5, which is shown to be an active filter, whose input terminal is connected to the wiper of potentiometer 3, and whose output terminal is connected to the inverting input terminal of inverting amplifier 4 through resistor 6.
In the active equalizer shown in FIG. 1, when the wiper of potentiometer 3 engages the resistance element thereof at its center tap, the active filter 5 is, in effect, disconnected from the circuit, and the frequency response of the active equalizer becomes flat. When the wiper is moved along the resistance element of potentiometer 3 toward input terminal T.sub.1, the range of frequencies of the input signal determined by filter 5 is boosted by the active equalizer, and when the wiper moves towards output terminal T.sub.2, such frequency range of the input signal is attenuated. It is also known in the prior art that, by selected first and second resistors 1 and 2 to have equal resistance values, the gain of inverting amplifier 4 can be made to be unity.
In practice, an active equalizer of the type generally shown in FIG. 1 is arranged as shown in FIG. 2, and in which a plurality of sub-circuits comprised of potentiometers 3A, 3B, 3C, . . . , active filters 5A, 5B, 5C, . . . , and resistors 6A, 6B, 6C, . . . , respectively, are connected in parallel in conjunction with a common first resistor 1, second resistor 2 and inverting amplifier 4 to form the active equalizer circuit.
In the prior art active equalizer shown in FIG. 1, and in which potentiometer 3 is disposed at the input side of active filter 5, any noise generated by an active element in active filter 5, such as a transistor or the like, is applied to inverting amplifier 4 with a constant level, regardless of the level of the input signal received by active filter 5. Therefore, the signal-to-noise ratio of the signal received at the inverting input terminal of inverting amplifier 4 deteriorates as the level of the input signal to active filter 5 is reduced.
In the case of the prior art active equalizer shown in FIG. 2, it is possible that an input signal may be applied through only some, and not all of active filters 5A, 5B, 5C. However, the noise produced by all of the active filters 5A, 5B, 5C, . . . , is nevertheless supplied to the input terminal of inverting amplifier 4, and cumulated. As a result, as the number of active filters in such an arrangement increases, the signal-to-noise ratio of the input signal provided to inverting amplifier 4 may deteriorate accordingly.